The present invention relates to a transmission method and system using a standard transmission network for connecting together elements of a seismic device, allowing to transmit with precision a signal indicative of a reference instant.
The transmission system according to the invention can find applications in many fields where remote stations are to be synchronized without necessarily using dedicated communication means.
This is the case in the field of seismic prospecting where signals received by seismic receivers arranged at the surface for example and reflected by the subsoil discontinuities in response to the emission of seismic waves by a vibratory or impulsive source are recorded. Seismic data collection sets comprising a great number of acquisition units arranged on the site to be explored and suited to collect (amplify, filter, digitize and store) the signals are used. The data stored are transmitted to a central control and recording station from each acquisition unit at fixed intervals (for example after each emission-reception cycle, after each daily session, etc.) or  less than  less than with the stream  greater than  greater than , as soon as a transmission time interval is available, either directly or by means of intermediate stations or concentrators. Seismic acquisition systems are for example described in patents FR-2,511,772 (U.S. Pat. No. 4,583,206) or FR-2,538,194 (U.S. Pat. No. 4,628,494), FR-2,692,384 (U.S. Pat. No. 5,550,787), FR-2,696,839 (U.S. Pat. No. 5,706,250), FR-2,710,757 (U.S. Pat. No. 5,563,847), FR-2,720,518 (U.S. Pat. No. 5,822,273), FR-2,766,580.
It is important that all the acquisition units distributed over the explored site can be synchronized with a common reference instant, generally the time of triggering of the seismic source, whatever the distance in relation to the central station that transmits the indicative signal thereto and whatever the transmission channel used therefore, failing which the combinations of centralized data become very imprecise.
There are well-known methods and devices allowing perfect time resynchronization of remote stations with a central station communicating by means of communication channels when it has been possible to measure the time of propagation of the signals on these channels.
Patent FR-2,538,194 (U.S. Pat. No. 4,628,494) filed by the applicant describes for example a method for synchronizing the acquisition of seismic signals by an acquisition unit in the field, with a reference instant such as the time of triggering of a seismic source when the time of propagation xcex94t thereto of the signal indicative of this instant is known, by a transmission channel such as a cable or a radio channel. The method essentially consists in sending a presignal starting the acquisition of signals coming from the seismic receivers and the storage thereof in a local memory. When the reference signal emitted thereafter is received at the acquisition unit at a time t, all the samples stored from the time (txe2x88x92xcex94t), i.e. the reference instant, are sought in the local memory.
Patent application FR-98/15,792 describes a method and a device also allowing to produce, for each seismic signal, a series of samples of these signals re-staggered from a reference instant, from a first series of digitized samples of this seismic signal produced from any initial instant prior to the reference instant, knowing the effective time interval between these two instants. The method comprises determining coefficients of a digital filter that will compensate for the fractional part of the measured effective time interval, and applying this digital compensation filter to the first series of samples, allowing to obtain a series of digitized samples re-staggered from the reference instant.
However, these known location and readjustment techniques can only be applied if the time of propagation of reference signals through the emitting and receiving stations is precisely known, transfer management being performed by software means, notably within the scope of a multitask management, especially if one considers that the uncertainty relative to the effective propagation time in a seismic transmission system such as those mentioned above for example should not exceed about 100 xcexcs.
Transmission without more or less random time lag is possible and easier to implement when one owns the network used and controls the form of the signals emitted and the coding mode. When signal TB representative of the effective time of emission by the source reaches the central station, it is possible, by means of suitable circuits, to inject this signal directly onto the communication channel that connects it to the receiving station and to prevent possible non-constant delays due to the station management system.
Standard communication networks are advantageous. They allow high-rate transmission, they are relatively easy to use and open-ended. They however work according to a particular communication protocol with specialized control circuits which one has to adapt to for precise transmission of the signals representative of reference instants.
The transmission method according to the invention allows to use a standard communication network for transferring synchronization data between a first control station and a second remote station while avoiding any uncertainty relative to the effective time of propagation of reference signals between communicating stations.
The transmission method according to the invention allows to connect together, by transmission channels, elements of a seismic device comprising local seismic data acquisition, processing and transmission units, a central control and recording station and at least one intermediate local control and concentration station, the elements of the device being provided with multitask control sets (such as programmed microcomputers), the method allowing precise synchronization of the local units with a reference signal emitted by the central station (a signal concomitant with the triggering of a seismic source for example).
The method comprises:
using a transmission network (preferably a local network, of Ethemet(trademark) type or others) working according to a particular transmission protocol, in order to interconnect the central station and at least one terminal station (the local intermediate stations or possibly the field acquisition units for configurations where the network extends to them),
previous measurement of the time of transit (propagation time) of the data on the transmission channels between the central station and the acquisition units,
pretriggering acquisition of seismic data by the acquisition units, the holding time interval in relation to the time of reception of the reference signal being fixed and greater than the transit time, and
applying to the reference signal, in the terminal station, an adjusted compensation delay taking account of the effective transfer time fluctuations linked with its passage through multitask control means of the terminal station and the local network, in order to respect this holding time interval.
According to a first embodiment (purely hardware type solution), the method comprises:
direct detection, in the central station, of the reference signal,
formation, from the reference signal detected, of a characteristic frame and direct application thereof to the interface module of the central station,
detection, by an interface module of each terminal station, of the characteristic frame of the reference signal coming in through the transmission channel, and
application to the reference signal in the terminal station of a compensation delay suited to said holding time interval.
According to a second embodiment (mixed hardware-software type solution), the method comprises:
direct detection of the reference signal and emission of a coded signal (a frame) to the terminal station by the local network,
measurement of a time interval between the time the reference signal is taken into account by the multitask control means of the central station and its effective transmission on the local network in form of a coded signal,
transmission of the value of this time interval in form of a second frame (coded signal), and
application to the reference signal in the terminal station of an adjusted compensation delay according to this time interval in order to respect said holding time interval.
The method is particularly flexible because it allows, by means of an adjustable transmission delay compensation, to readily take account of the various data transfer rates of the transmission channels available in a complex transmission system such as those used for example in modem seismic prospecting devices.
The transmission system according to the invention allows to connect together, by transmission channels, elements of a seismic device comprising local seismic data acquisition, processing and transmission units, a central control and recording station and at least one local control and concentration station, the elements of the device being provided with multitask control sets. It allows precise synchronization of the local units with a reference signal emitted by the central station (signal concomitant with the triggering of a seismic source for example).
The system comprises:
a transmission network, preferably a local network (of Ethernet type or others) working according to a particular transmission protocol, for interconnecting the central station and at least one terminal station (the local intermediate stations or possibly the field acquisition units if the network extends thereto), comprising at least one transmission channel whose transmission lag is known, associated with interface modules in the central station and each terminal station, suited to respect said particular transmission protocol,
means for measuring the time of transit (propagation time) of the data on each transmission channel to be used between the central station and the acquisition units,
means for pretriggering acquisition of the seismic data by the acquisition units, with a holding time interval in relation to the instant of reception of the reference signal which is fixed and greater than the transit time, and
means for applying to the reference signal an adjusted delay taking account of the effective transfer time fluctuations linked with its passage through said multitask control means (in the software  less than  less than layers greater than  greater than  of the programmed processors) and the local network, this delay being suited to the effective rate (measured when the device is installed in the field) of each transmission channel used.
This combination of a fixed holding time applied by all the acquisition units and of intermediate delay adjustment means suited to complete the known propagation times measured on the transmission channels, by means of a complementary delay taking account of the managing time fluctuations, allows all the acquisition units to be adjusted together to a common reference instant.
The time of transmission by the network being known with precision, it is also possible to combine such a network with conventional dedicated optical fiber, radio or cable transmission channels, while maintaining a perfect synchronism between these various channels.